Heat the oven to 190°C.
Butter a 13 by 9-inch ovenproof dish or lasagne dish and set aside.
Cook the egg noodles in salted boiling water according to package directions for al dente, about 8 minutes. Drain and immediately place the noodles in a large bowl filled with ice water to stop the cooking. Once cooled, pour the pasta into a large bowl with the tuna, peas and both cheeses. Toss to combine.
In a large pan with straight sides add the rest of the butter, the olive oil, thyme, and onions. Season with a sprinkle of salt and pepper and cook on medium heat until the onions are tender and translucent, about 5 minutes. Add the mushrooms, season with salt and cook gently until tender and darkened, about 5 minutes more. Add the Worcestershire sauce, horseradish, and sprinkle flour over the entire pan. Stir to help the flour to soak into vegetables and cook a minute more to lose the flour taste. Raise the heat to medium high and add the chicken stock. Cook until slightly thickened then slowly stir in the double cream. Simmer until the cream is thickened slightly, about 4 minutes more. Taste and season with salt, if needed. Pour the mushroom sauce over the prepared noodles in the large bowl and quickly stir to combine. Immediately pour into the prepared dish.
To make the topping: In a small bowl, combine the breadcrumbs and olive oil. Season with a sprinkle of salt and a grind or two of pepper. Stir until the crumbs soak up the oil evenly. Sprinkle the breadcrumb mixture evenly over the top of the dish and place uncovered in the oven until the sides are bubbly and the top is golden brown, about 35 minutes.